Future generations will marvel at the ridiculous ways we’ve been keeping warm. With a furnace, you’re inefficiently burning toxic, planet-warming gas. In a big city like New York, your building might have a boiler that burns oil or gas to heat water or produce steam, which feeds wheezing radiators in every unit. Electric-resistance space heaters are more efficient, but even they’re not nearly efficient enough, as you’ll see in your utility bill if you run one too much.

For many decades now, there’s been a far more efficient device that climate nerds love: the heat pump. Instead of generating heat like those other techniques, it transfers warmth from outdoor air into an indoor space using nifty tricks of physics, even when that outdoor air is freezing. A heat pump can attach to a home’s central ducting system, replacing a gas furnace, or ductless models can attach to walls.

Now, New York is experimenting with what could be an even more powerful kind of heat pump for urbanites: one that slips over a window sill. In 2022, NYC announced a $70 million investment for developing and producing 30,000 heat pumps for its public housing. One of the companies awarded funding, Gradient, rolled out 36 window units and collected data on their performance over this past winter. It’s now sharing early results. (You can see what Gradient’s device looks like at the top of this story. Think of it like a window AC unit but much more advanced.)

Gradient says that on the coldest days, its window heat pump performance indicates that the unit can reduce the cost to warm a New York home by between 15 and 55 percent compared to gas-fired steam heat, and by between 51 and 74 percent compared to oil-fired steam heat. On moderate days, projected savings are between 29 and 62 percent compared to gas, and 59 and 78 percent compared to oil. The hope is that heat pumps like these can help the city and the state transition away from fossil fuels; New York is part of a consortium of nine states aiming to get heat pumps to account for 65 percent of residential heating, AC, and water-heating shipments by 2030.

Because it’s moving heat instead of generating it, a fully electric heat pump is inherently far more efficient than burning fossil fuels. It can even be up to five times more efficient than an electric-resistance heater. So if we replace as many fossil-fuel heating systems as we can with heat pumps, we can run them on a grid increasingly loaded with renewable energy. Recent modeling found that if everyone in the US got a heat pump, it’d crash emissions in the residential sector by 36 to 64 percent.

A user also reduces energy consumption because a window unit heats only a single room at a time. A boiler, on the other hand, may be dumping too much heat into an apartment. A south-facing apartment, for example, will heat up more from solar radiation than a north-facing one would. If it’s sweltering inside because the building-wide system doesn’t account for this difference, and you have to open your window, you’re wasting that boiler-generated heat.

“There’s a massive difference in the amount of heat that our system is putting out when a user asks for heat to be comfortable versus a radiator which dumps tons of extra heat into the room,” says Vince Romanin, CEO of Gradient. “If they’re able to set that temperature on a per-room basis, not a per-building basis, you end up—because you’re only heating and cooling the rooms needed—with about 20 percent less energy use.”

The New York City Housing Authority says that residents are overall happy with the units, especially the ability to control temperatures. In the summer, a heat pump reverses to work like an air-conditioning unit. So people who’ve never had AC suddenly have a clean, efficient device that both heats and cools. “The heat pumps allow NYCHA to move away from natural-gas-based steam heating systems and are also two to six times as energy efficient as these systems,” says Shaan Mavani, chief asset and capital management officer of the Housing Authority.

With these heat pumps, New York is inverting the usual pattern for new energy technology, which is usually too expensive for regular people to afford. “It’s relatively cheap, relatively simple technology that’s plug-and-play, that works in the 100-year-old public-housing brick building,” says climate economist Gernot Wagner of the Columbia Business School. “It’s the rich who are supposed to be early adopters of the new, sexy, top-of-the-line climate tech.”

Gradient’s all-weather heat pump, meant to operate in colder climates, is set to be priced at $3,800 later this year. That’d be offset by a growing number of state and federal rebates and tax credits that encourage decarbonization. With a full-on heat pump system working through ducting in a fancy person’s home, you’re looking at the costs of potentially having to upgrade your electric system to handle the additional power demand, whereas a smaller window version just plugs into the wall. Actually installing a heat pump isn’t much different from installing a typical AC unit, usually taking about a day, but the technician will need some special training to do it. (In general, the US is desperately short of the skilled workers available to install enough heat pumps and other green tech to decarbonize fast enough.) By contrast, you can install a window-sill heat pump in under an hour, Gradient says.

One of the hurdles for urban apartment dwellers is the potential for an operational cost shift: If the landlord had been paying for a central steam heating system, and the renter is now running a heat pump on their own unit’s electricity, their bills may increase. Some 90 percent of the New York City Housing Authority’s residents live in buildings that are “master metered” anyway, meaning they don’t pay individual electric bills. For the remaining 10 percent, the NYCHA will likely introduce a utility allowance to ensure that the switch to a heat pump doesn’t increase expenses. At the same time, as residents make that switch, the agency will save on the costs associated with repairing the existing heating distribution systems. “The heat pumps obviate the need for these investments,” says Mavani.

What the NYCHA has embarked on is a plan that other metropolises could copy for switching their own multifamily buildings to heat pumps. “That said, every city has a different mix of building typologies, local codes, heating and cooling needs, and construction and utility costs,” says Mavani. “Hopefully, based on the experience in New York, other multifamily building owners—whether public or private—will have better data points to support their own decisionmaking.”

Heat pumps will only get cheaper from here. Unlike stagnant fossil-fuel heating techniques, heat pumps are a technology that’s evolving, getting more and more efficient at extracting heat from outdoor air and moving it inside. “Heat pumps are the classic example of a technology that over time will only get better, will only get cheaper,” says Wagner. “We know where we need to go. We have to electrify buildings; we have to get off gas and oil heat especially. This is the way to do that.”